Advanced device elements such as integrated circuits, semiconductor lasers, active matrix type liquid crystal display panels and solar battery panels are made by successive fabrication of specific film layers and the like on a highly cleaned surface of substrate materials such as silicon, gallium arsenide and glass.
Extreme precision is required for manufacturing of such device parts, and if the wafer materials to be processed are contaminated with adhesion or adsorption of even microscopic contaminating impurities, it becomes difficult to manufacture high quality products.
Further, such substrate wafers are susceptible to accumulating static charges during the processes of transport and various treatments, resulting in their attracting and holding contaminations from the surrounding atmosphere.
For example, if moisture as impurity is adhering to the surface of the semiconductor wafer, it will cause problems during the subsequent device processing. If there is excess moisture adhering to the surface, and the surrounding atmosphere contains a partial pressure of oxygen, a native oxide film is formed on the surface of the wafer, interfering with subsequent fabrication steps such as a deposition of a desired thin film.
Further, during the process of making thin film transistors (TFT) used in liquid crystal display panels, for example, if moisture is present on an electrically insulating film, such as SiN.sub.x, it interferes with the process of forming a uniform and controlled thickness of amorphous silicon (a-Si) film.
Further, during the process of fabricating gate oxide films, for example, if moisture is present on the surface of an n- or p-region, a SiO.sub.x film is produced at the interface between a SiO.sub.2 film and a Si substrate, and such a MOS transistor would not function as a proper switch. Similarly, in the process of fabricating capacitor electrodes, if moisture is present on the surface of a capacitor, a SiO.sub.x film is produced on its surface, and electrical charge storage capability of the capacitor is damaged, and its ability to function as a memory element is destroyed.
In metallization process, a titanium nitride (TiN) film is deposited before forming a tungsten film to prevent spikes caused by the presence of tungsten silicide, and if moisture is present on the Si substrate, the adherence of the interface between Si substrate and TiN is diminished.
Other contaminating substances other than water can also affect the device performance, for example, if heat treatment is carried out in the presence of organic impurities, such as methane, carbon can react with the surface of the silicon substrate to form a SiC film, and device performance is degraded.
For these reasons, various process chamber used in manufacturing of such integrated semiconductor devices are placed in clean rooms capable of filtering dust particles which are usually comprised of microparticles.
Integrated semiconductor circuits are produced after a series of successive fabrication steps, and therefore, the in-process substrate wafers are subjected to a number of processing steps in various process chamber, and are also subjected to a process of being transported from one processing line to another processing line.
During such transporting steps, the wafers are exposed to the atmosphere in the clean rooms which are usually kept at a temperature between 20-25.degree. C., and a relative humidity of about 50%, and although microparticles are filtered out, many gaseous particles are still present. As a result, some impurities present in the clean room can be adsorbed on the surface of the wafers. For example, moisture present in the air atmosphere is instantly adsorbed on the substrate surface. To prevent such moisture adsorption, it is not a practical solution to remove moisture from all the clean rooms.
A proposal has therefore been made to transport the wafers from a process chamber to another process chamber while holding them in pockets, of a transport robotic device, filled with dry nitrogen. The disadvantage of this method is that transport process is lengthy and productivity is low.
There has been a proposal to connect one process chamber and another process chamber through a tunnel, which is filled with some inert gas such as nitrogen, and transport the wafers through the gas-filled tunnel (refer to Japanese Patent Application, First Publication, H5-211225).
Using this system, it becomes possible to transport the wafers from one process chamber to another process chamber without exposing the wafers to external atmospheres. Such a system also enables to utilize the power of flowing gas stream for transporting the wafers.
There has also been an improved version of the system disclosed above involving recycling of the inert transport gas in the tunnel, by treating it in a Cryogenic nitrogen generator (see, T. Ohmi et al., Breakthrough for Scientific Semiconductor Manufacturing in 2001, No. 21, 1992).
However, even in an apparently fool-proof system, it is unrealistic to expect that unanticipated entry of outside atmosphere or entry of outside atmosphere, through an accidental breakage in the tunnel, cannot occur. Even in an inert-gas filled tunnel, it is difficult to always filter out all the particles, particularly moisture particles, from the tunnel environment.
Therefore, if the moisture concentration or partial pressure of water in the inert gas tunnel atmosphere should rise for some reason, adsorption of water on the substrate wafer can take place, leading to the formation of a native oxidation film, a loss in production yield and a potentially serious revenue loss.
Also, if an accidental break should occur in the tunnel, the entire production lines must be stopped to examine the causes of failure and accident site, and to undertake repair work, leading to a prolonged downtime and production losses.
Furthermore, during such an accidental breakage in the tunnel, it should be considered that along with inflow of outside atmosphere into the tunnel, the inert gas atmosphere inside the tunnel would flow out of the tunnel. Should a large quantity of inert gas escape into the surrounding environment, it could lead to oxygen deficiency in the surrounding areas.
That was why there has been a proposal to run oxygen gas with inert gas previously in the inert gas tunnel, to prevent occurrence of oxygen deficiency even if there is an accident.
However, this action would promote the formation of native oxide films on the substrate surface.
Additionally, within the inert gas tunnel, there is a potential contamination from back streaming of process gases used in various process chamber, for example, H.sub.2 O, CO.sub.2 and organic group gases such as CH.sub.4 (shortened to H--C hereinbelow) which are potential contaminations to the wafers being transported in the inert gas tunnel.
Out of the background of such developments, a consideration has been given to an idea that a cleaning chamber for cleaning the surface of the wafers be placed between a process chamber and the tunnel in order to avoid producing rejects even if unwanted substances such as moisture are happen to be present within the tunnel. By having such a cleaning chamber, it can be ensured that the substrate surfaces can always be maintained clean and proper processing can be performed, even if moisture and other contaminations are adhered to the substrate surface, by carrying out certain cleaning steps before performance of the next processing line.
Such cleaning chambers may be based on high temperature heating (about 300.degree. C.) of wafers to remove water adhering to the surface of the wafers. Other processes include removing of contaminations, water and native oxide film, by activating the substrate surface with plasma cleaning or ion cleaning.
However, the thermal technique is not recommended for TFT wafers because of possible shape distortion of the wafers, and this technique is not sufficient for removing native oxide films from the wafers, at such a low temperature of 300.degree. C. If the heating temperature is lowered to avoid a heat distortion, the process of moisture removal becomes excessively lengthy or the results are inadequate.
The plasma cleaning and ion cleaning methods present not only the inherent problem that the surface cleanliness would be inadequate unless the energy or time of exposure is sufficient, but even more seriously, if such cleaning processes which are high power for the case of many adhered contaminations or formation of native oxide film, are applied to a surface which is already sufficiently clean, the substrate surface may become rough, and even more serious damage may be inflicted on the substrate surface.